1. Field of the Invention
The present invention relates to a scanning acoustic microscope of transmission type for forming an image with the use of ultrasound transmitted through a sample.
2. Description of the Related Art
A reflection type scanning acoustic microscope is known in the art which can observe a sample image, sample defect, etc., through the utilization of echo waves coming from a sample. This type of microscope cannot achieve a high image resolution when a sample is low in its reflectivity, such as a living cell. A transmission type scanning acoustic microscope has been conceived in the art which utilizes a contrast resulting from the absorption of ultrasound by a sample when it has been transmitted through the sample.
FIG. 4 shows one form of a transmission type scanning acoustic microscope.
In the transmission type scanning acoustic microscope, a transmission-side acoustic lens 1 and reception-side acoustic lens 2 are so arranged as to have their optical axes aligned with each other, the acoustic lens 1 being adapted to send ultrasound to a sample S and the acoustic lens 2 being adapted to detect ultrasound which has been transmitted through the sample S.
The positions and attitudes of these acoustic lenses 1 and 2 have to be so adjusted as to have their focal points and their optical axes aligned with each other, respectively. For this reason, one (the acoustic lens 2) of these acoustic lenses is fixed and the other acoustic lens 1 is supported by a known positioning/tilt adjusting mechanism 3. The sample S is located at a confocal point 0 between the acoustic lenses 1 and 2.
The sample S is placed on the surface of a thin film (not shown) having a uniform thickness, such as a Mylar sheet and loaded at the confocal spot 0. The thin film Mylar sheet is placed on a transfer mechanism 4 such that its basic end is movable in a direction of an optical axis. The transfer mechanism 4 is so held by an X-Y scanner 5 so that it can be moved in a plane perpendicular to the optical axis.
A high frequency signal for ultrasound generation is supplied to a transducer 6 which is provided on the transmission side acoustic lens 1. The ultrasound which is generated through its electric/acoustic conversion by a transducer 6 is converged by the transmission-side acoustic lens 1 at a spot where the sample S is located. By so doing, the ultrasound coming from the acoustic lens 1 is directed to the sample S through a coupler liquid 7.
Since the sample S is located at a focal spot of the reception-side acoustic lens 2, the ultrasound enters the reception-side acoustic lens 2 after it has been transmitted through the sample S.
Through the two-dimensional scanning of the sample S by the X-Y scanner 5, resultant ultrasound is acoustric/electric converted by a transducer 8 provided on the reception-side acoustic lens 2, generating an acoustic/electric conversion signal as an output signal. The output signal reveals a predetermined contrast corresponding to the inner structure of the sample in accordance with the level of absorption involved when the ultrasound is transmitted through the sample. By detecting the contrast data of the output signal it is possible to obtain an image corresponding to the sample S.
In the aforementioned confocal type scanning acoustic microscope, unless the two focal spots of the two acoustic lenses 1 and 2 have their positions aligned more accurately than their image resolution it is not possible to achieve a high degree of measurement of the order of the wavelength of the ultrasound. The resultant measurement is such that
the plane resolution d.sub.XY =1.0.times.F.times..lambda.=3.7 .mu.m; and
the depth resolution dz=2.times.F.sup.2 .times..lambda.=7.5 .mu.m where
F number: 1.0
the working frequency of the acoustic
lenses 1, 2: 400 MHz
the coupler liquid 7: water.
It is required that the actual accuracy be empirically an order of magnitude greater than the above.
Thus much more accuracy as set out above is required in the three-dimensional positioning/tilt adjustment mechanism 3 in the conventional transmission type scanning acoustic microscope. Further, the positioning operation has been very difficult because high accuracy has to be maintained during scan.